A number of meiotic mutants (mutants in which meiotic recombination and/or chromosome segregation is abnormal) have been isolated (Sandler et al. 1968; Baker and Carpenter 1972) in Drosophila. Mutants of 14 loci (18 mutants) are recombination defectives; all exhibit decreased frequencies of meiotic recombination. To inquire whether the processes of meiotic and mitotic recombination have any steps in common, we have begun an examination of spontaneous mitotic crossing over in the presence of the recombination defective meiotic mutants. Preliminary data suggest that: (1) many of these mutants increase the frequency of somatic crossing over; (2) frequent breakage of somatic chromsomes occurs in the presence of mutants at one, and possibly two other loci; and (3) mutants at one of the latter loci may be hypersensitive to killing by X-rays. These data suggest meiotic and mitotic recombination are not under common genetic control, but that some of the enzymes that mediate meiotic recombination are also required in mitotic cells by the maintenance of chromosomal integrity. Experiments are planned to further test these notions and to examine in detail the somatic effects of these and other recombination defective mutants. Hopefully these experiments will lead towards the identification of the enzymatic defects in these recombination deficient mutants. In addition: (1) Mutants defective in the process of sex determination are being studied to sequence the steps they specify and the developmental time at which these loci act. (2) The Minute loci are being studied to test biochemical and genetic predictions of the hypothesis that they are the structural loci for the ribosomal proteins rather than the tRNA's.